[cmake-developers] Issues with overriding Python version, possible bug?

[Alan W. Irwin]

On 2017-05-01 08:54-0400 Brad King wrote:

> On 04/29/2017 04:47 PM, Alan W. Irwin wrote:
>> looks for Python 3, but if that does not exist it looks for Python 2.
>>
>> find_package(PythonInterp 3)
>> find_package(PythonInterp 2)
>>
>> So far so good, but I am sure there will be some of our users that
>> prefer Python 2 even when Python 3 is installed on their system.
>
> You should use your own variable for that.  The version-named executables
> are searched based on PythonInterp_FIND_VERSION which comes from the
> version number passed to find_package.  The order of find_package calls
> and the version numbers passed to them is controlled by your project,
> so if you want a switch for this it should be in your project.
>
> Or, you can tell the users to pass `-DPYTHON_EXECUTABLE=/path/to/python2`
> to specify the desired python executable.

Thanks, Brad, for that guidance.  I have decided to go with our own
FORCE_PYTHON2 option for this purpose.

>
>> way (e.g., by setting Python_ADDITIONAL_VERSIONS) to specify the
>> Python version they prefer.
>
> That variable is only for aiding the search for future python versions
> not released as of the running version of CMake, e.g. 3.12.

Actually, for the

find_package(PythonInterp)

case, i.e., when no version specified to find_package, it does allow
the user to pick Python2, for example.  This is exactly consistent with the
current documentation of Python_ADDITIONAL_VERSIONS which states:

The Python_ADDITIONAL_VERSIONS variable can be used to specify a list
of version numbers that should be taken into account when searching
for Python.  You need to set this variable before calling
find_package(PythonInterp).

Which is why I thought I might have found a bug when it failed to work
when find_package specified the version.  On the other hand, the name
of the variable does support your interpretation.  So perhaps a change
to the real documentation is all that is needed here.  I suggest
adding this sentence to the above:

Python_ADDITIONAL_VERSIONS is ignored if find_package specifies a version.

Alan
__________________________
Alan W. Irwin

Astronomical research affiliation with Department of Physics and Astronomy,
University of Victoria (astrowww.phys.uvic.ca).

Programming affiliations with the FreeEOS equation-of-state
implementation for stellar interiors (freeeos.sf.net); the Time
Ephemerides project (timeephem.sf.net); PLplot scientific plotting
software package (plplot.sf.net); the libLASi project
(unifont.org/lasi); the Loads of Linux Links project (loll.sf.net);
and the Linux Brochure Project (lbproject.sf.net).
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Linux-powered Science
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